“Never memorize what you can look up in books” is a quote often attributed to Einstein, though what he actually said was somewhat different. He was asked, but did not know the speed of sound as included in the Edison Test. When this was pointed out, he said, “[I do not] carry such information in my mind since it is readily available in books. He also said, “…The value of a college education is not the learning of many facts but the training of the mind to think.” See also Ray Bradbury, in Fahrenheit 451:
“Cram them full of non-combustible data, chock them so damned full of ‘facts’ they feel stuffed, but absolutely ‘brilliant’ with information. Then they’ll feel they’re thinking, they’ll get a sense of motion without moving. And they’ll be happy, because facts of that sort don’t change. Don’t give them any slippery stuff like philosophy or sociology to tie things up with. That way lies melancholy.”
The single thing I’ve found it valuable to memorize is poetry. As a child I learned hundreds of poems by heart, which I can recite even now. I wanted to become a writer, and felt that poetry was perfected language, so having it in my subconscious mind would make the music of language always available to me.